Papillomaviruses infect a wide variety of different species of animals including humans. Infection is typically characterized by the induction of benign epithelial and fibro-epithelial tumors, or warts at the site of infection. Each species of vertebrate is infected by a species-specific set of papillomavirus, itself comprising several different papillomavirus types. For example, more than sixty different human papillomavirus (HPV) genotypes have been isolated. Papillomaviruses are highly species-specific infective agents. For example, canine and rabbit papillomaviruses cannot induce papillomas in heterologous species such as humans. Neutralizing immunity to infection against one papillomavirus type generally does not confer immunity against another type, even when the types infect a homologous species.
In humans, papillomaviruses cause genital warts, a prevalent sexually-transmitted disease. HPV types 6 and 11 are most commonly associated with benign genital warts condylomata acuminata. Genital warts are very common, and subclinical or inapparent HPV infection is even more common than clinical infection. While most HPV-induced lesions are benign, lesions arising from certain papillomavirus types, e.g., HPV-16 and HPV-18, can undergo malignant progression. Moreover, infection by one of the malignancy-associated papillomavirus types is considered to be a significant risk factor in the development of cervical cancer, the second most common cancer in women worldwide. Of the HPV genotypes involved in cervical cancer, HPV-16 is the most common, being found in about 50% of cervical cancers.
In view of the significant health risks posed by papillomavirus infection generally, and human papillomavirus infection in particular, various groups have reported the development of recombinant papillomavirus antigens and their use as diagnostic agents and as prophylactic vaccines. In general, such research has been focused toward producing prophylactic vaccines containing the major capsid protein (L1) alone or in combination with the minor capsid protein (L2). For example, Ghim et al, Virology, 190:548-552 (1992), reported the expression of HPV-1 L1 protein, using a vaccinia expression in Cos cells, which displayed conformational epitopes and the use thereof as a vaccine or for serological typing or detection. This work is also the basis of a patent application, U.S. Ser. No. 07/903,109, filed Jun. 25, 1992 (abandoned in favor of U.S. Ser. No. 08/216,506, filed on Mar. 22, 1994), which has been licensed by the assignee of this application. Also, Suzich et al, Proc. Natl. Acad. Sci., U.S.A., 92:11553-11557 (1995), report that the immunization of canines with a recombinant canine oral papillomavirus (COPV) expressed in a baculovirus/insect cell system completely prevented the development of viral mucosal papillomas. These results are important given the significant similarities between many HPVs and COPV. For example, COPV, similar to HPVs associated with anogenital and genital cancer, infects and induces lesions at a mucosal site. Also, the L1 sequences of COPV shares structural similarities to HPV L1 sequences. Given these similarities, the COPV/beagle model is useful for investigation of L1 protein-containing vaccines, e.g., investigation of the protective immune response, protection from natural infection and optimization of vaccination protocols. (Id.)
Also, a research group from the University of Rochester reported the production of human papillomavirus major capsid protein (L1) and virus-like particles using a baculovirus/insect cell expression system (Rose et al, University of Rochester, WO 94/20137, published on Sep. 15, 1994). In particular, they reported the expression of the L1 major capsid protein of HPV-6 and HPV-11 and the production of HPV-6, HPV-11, HPV-16 and HPV-18 virus-like particles.
Further, a University of Queensland research group also purportedly disclosed the recombinant manufacture of papillomavirus L1 and/or L2 proteins and virus-like particles as well as their potential use as vaccines (Frazer et al, WO 93/02189, published Feb. 4, 1993).
Still further, a United States government research group reported recombinant papillomavirus capsid proteins purportedly capable of self-assembly into capsomere structures and viral capsids that comprise conformational antigenic epitopes (U.S. Pat. No. 5,437,951, Lowy et al, issued Aug. 1, 1995). The claims of this patent are directed to a specific HPV-16 DNA sequence which encodes an L1 protein capable of self-assembly and use thereof to express recombinant HPV-16 capsids containing said HPV-16 L1 protein.
With respect to HPV capsid protein containing vaccines, it is widely accepted by those skilled in the art that a necessary prerequisite of an efficacious HPV L1 major capsid protein-based vaccine is that the L1 protein present conformational epitopes expressed by native human papillomavirus major capsid proteins (see, e.g., Hines et al, Gynecologic Oncology, 53:13-20 (1994); Suzich et al, Proc. Natl. Acad. Sci., U.S.A., 92:11553-11557 (1995)).
Both non-particle and particle recombinant HPV L1 proteins that present native conformational HPV L1 epitopes have been reported in the literature. It is known that L1 is stable in several oligomeric configurations, e.g., (i) capsomeres which comprise pentamers of the L1 protein and (ii) capsids which are constituted of seventy-two capsomeres in a T=7 icosahedron structure. Also, it is known that the L1 protein, when expressed in eukaryotic cells by itself, or in combination with L2, is capable of efficient self-assembly into capsid-like structures generally referred to as virus-like particles (VLPs).
VLPs have been reported to be morphologically and antigenically similar to authentic virions. Moreover, immunization with VLPs has been reported to elicit the production of virus-neutralizing antibodies. More specifically, results with a variety of animal papillomaviruses (canine oral papillomavirus and bovine papillomavirus-4) have suggested that immunization with VLPs results in protection against subsequent papillomavirus infection. Consequently, VLPs composed of HPV L1 proteins have been proposed as vaccines for preventing diseases associated with human papillomavirus infections.
For example, it has been reported that the L1 protein can assemble into VLPs when expressed using recombinant baculovirus and vaccinia virus vectors and in recombinant yeast (Hagensee et al, J. Virol., 68:4503-4505 (1994); Hofmann et al, Virology, 209:506-518 (1995); Kirnbauer et al, Proc. Natl. Acad. Sci. USA, 89:12180-12184 (1992); Kirnbauer et al, J. Virol., 67:6929-6936 (1993); Rose et al, J. Virol., 67:1936-1944 (1993); Sasagawa et al, Virology, 206:126-135 (1995); Suzich et al, Proc. Natl. Acad. Sci. USA, 92:11553-11557 (1995); Volpers et al, Virology, 200:504-512 (1994); Zhou et al, J. Virol., 68:619-625 (1994)).
Most previous recombinant L1 preparations isolated from eukaryotic cells have resulted in a variable population of VLPs approaching 55 nm in diameter, which are similar in appearance to intact virions. However, VLP assembly is somewhat sensitive to cell type. For example, L1 expressed in Escherichia coli is expressed largely in the form of capsomeres or smaller, with few or no capsids apparent either in the cell or upon purification (Rose et al, J. Virol., 67:1936-1944 (1993); Li et al, J. Virol., 71:2988-2995 (1997)). Similar results are observed when the polyoma virus VP1 protein is expressed in E. coli (Salunke et al, Biophys. J., 56:887-900 (1989)).
To date there has not been reported an effective in vitro method for the quantitative disassembly and subsequent reassembly of papillomavirus VLPs. Such a method would be highly advantageous as it would potentially enable the preparation of more stable and/or homogeneous papillomavirus VLPs. This would be beneficial as homogeneity and stability are both significant concerns in vaccine preparation and characterization during manufacture. Furthermore, the ability to disassemble and reassemble VLPs has important applications to VLP purification. HPV L1 proteins expressed in eukaryotic cells spontaneously assemble to form VLPs, as discussed above. However, most protein purification prodecures have been designed to purify proteins much smaller than the .about.20 million dalton, 55 nm VLP. The potential to disassemble VLPs extracted from eukaryotic cells to the level of L1 capsomeres or smaller, purify the smaller components by conventional techniques, and then reassemble to form VLPs at the desired stage of the purification process is very powerful, and is currently being utilized in the purification of HPV-16.sub.Tr VLPs, as discussed below (composed of a mutated form of the HPV-16 L1 protein from which the C-terminal 34 amino acids have been deleted). Finally the ability to disassemble and reassemble VLPs in vitro allows for the packaging of desired exogenous compounds within the reassembled VLP.
Earlier attempts at papilloma VLP disassembly have included experiments based on earlier work performed on polyomavirus, a related papovavirus, wherein it was shown that both the reduction of disulfides and chelation of cations were essential for virion disassembly (Brady et al, J. Virol., 23:717-724 (1977)). However, in the case of HPV VLPs it has been shown that the low levels of reducing agent (1-10 mM DTT) which provide for optimal polyomavirus disassembly in the presence of low levels of chelating agents (e.g., 0.5-10 mM EGTA) were only slightly effective at disassembly of papillomavirus VLPs (see Table 1, Li et al, J. Virol., 71:2988-2995 (1997)). By contrast, partially trypsinized HPV-11 L1 VLPs have been reported to disassociate effectively under such conditions (Li et al, J. Virol., 71:2988-2995 (1997)). However, this is disadvantageous as the use of protease may result in adverse effects, e.g., removal of neutralizing epitopes.
Also, Sapp and coworker demonstrated that "partial disassembly" of HPV-33 VLPs could by achieved by treatment with reducing agent alone (20 mM DTT). However, the extent of VLP breakdown was not determined (Sapp et al, J. Gen. Virol., 76:2407-2412 (1995)).
As discussed above, HPV capsid assembly requires correctly-folded L1 protein. However, additional factors significant for VLP formulation and stability have not been well elucidated. With respect thereto, it is generally known that VLP assembly can be affected by numerous factors. For example, factors and conditions known to affect assembly for other viruses include, by way of example: pH, ionic strength, post-translational modifications of viral capsid proteins, disulfide bonds, and divalent cation bonding, among others. For example, the importance of cation bonding, specifically calcium, in maintaining virion integrity has been shown for polyomavirus (Brady et al, J. Virol., 23:717-724 (1977)), an rotovirus (Gajardo et al, J. Virol., 71:2211-2216 (1997)). Also, disulfide bonds appear to be significant for stabilizing polyomavirus (Walter et al, Cold Spring Har. Symp. Quant. Biol., 39:255-257 (1975); Brady et al, J. Virol., 23:717-724 (1977)); and SV40 viruses (Christansen et al, J. Virol., 21:1079-1084 (1977)). Also, it is known that factors such as pH and ionic strength. influence polyomavirus capsid stability, presumably by affecting electrostatic interactions (Brady et al, J. Virol., 23:717-724 (1977); Salunke et al, Cell, 46:895-904 (1986); Salunke et al, Biophys. J., 56:887-900 (1980)). Also, it is known that post-translational modifications of some viral capsid proteins may affect capsid stability and assembly, e.g., glycosylation, phosphorylation, and acetylation (Garcea et al, Proc. Natl. Acad. Sci. USA, 80:3613-3617 (1983); Xi et al, J. Gen. Virol., 72:2981-2988 (1991)). Thus, there are numerous interrelated factors which may affect capsid stability, assembly and disassembly which vary widely even for related viruses.
Therefore, there exists a need in the art for elucidation of the factors that affect papillomavirus VLP assembly and disassembly. Moreover, based thereon, there exists a need in the art for an efficient in vitro method of disassembly and reassembly of papillomavirus VLPs which results in VLPs having good homogeneity, stability, and immunogenic properties, i.e., those which present conformational and more particularly neutralizing epitopes expressed on the surface of native, intact papillomavirus virions. Moreover, there is a significant need for methods for disassembly and reassembly of papillomavirus VLPs which obviate the problems of partial VLP disassembly and which avoid the use of protease used in prior methods of generating papillomavirus VLPs.